1. Field of the Invention
The invention relates in general to a method ore removing carbon contamination on a semiconductor substrate, and more particular, to a method of removing carbon contamination by forming a sacrificial oxide layer.
2. Description of the Related Art
An etching process is to remove a part of a thin film with a thickness from hundreds to thousands angstrom by chemical reaction or physical method. A photolithography process is performed first to transfer a pattern on a photo-mask to the thin film. The etching process is then performed to define the thin film.
Two kinds of etching processes, including a wet etching and a dry etching, have been widely applied in semiconductor fabrication technique. The wet etching process is basically an isotropic etching process, so that an undercut is often caused under a photo-resist layer. The undercut may cause problems in electrical properties in a device formed subsequently. A very precise pattern can typically be obtained by dry etching since the dry etching process is a kind of anisotropic etching process.
However, though a more precise patterned may be transferred by dry etching, the carbon containing etching plasma such as a gas mixture of carbon fluoride (CF.sub.4), carbon tri-flouride (CF.sub.3), and argon may easily contaminate an exposed semiconductor substrate or silicon layer with carbon. That is, the silicon atoms of the exposed semiconductor substrate or silicon layer may combine and bond with the carbon atoms. As a consequence, the thickness and the quality of an oxide layer formed subsequently is difficult to control.
In FIG. 1, a flowchart showing a conventional method of removing carbon contamination on a semiconductor substrate is presented. When a large quantity of carbon contamination is left on a semiconductor substrate, hydrogen fluoride is commonly used as an etchant to remove the carbon contamination. However, the effect of removal by this method is not good. A residue of carbon contamination is typically left on the semiconductor substrate even after the etch process with diluted hydrogen fluoride as an etchant. Further, if the etch is performed for a long time in order to remove the carbon contamination completely, the devices or other structures formed on the semiconductor substrate are likely to be damaged by the corrosive hydrogen fluoride. Thus, the hydrogen fluoride etch is typically limited and carbon contamination remains.
To resolve the problem of the method shown in FIG. 1, another prior technique was developed. FIG. 2 shows a flow chart of another conventional method of removing carbon contamination on a semiconductor substrate. When a large quantity of carbon contamination is left on a semiconductor substrate, a sacrificial oxide layer with a thickness of about 100 .ANG. is formed on the substrate using thermal oxidation. The sacrificial oxide layer is then stripped. The function of the sacrificial oxide layer is to trap carbon contamination. Thus, when the sacrificial oxide layer is stripped, the trapped carbon is removed. However, as mentioned above, carbon contamination inhibits the growth of oxide, and therefore, the effect of trapping carbon contamination in the oxide layer is limited. Further, stripping the sacrificial oxide layer, a residue of carbon contamination still remains. Moreover, it takes a long time to grow a thick oxide layer on a carbon contamination wafer, and that slows down production.
A detailed description of the method shown in FIG. 2 is explained as follows with accompanying drawings FIG. 3A to FIG. 3D. In FIG. 3A, a semiconductor substrate 300 is provided. On the semiconductor substrate 300, a large quantity of carbon contamination 302 is left by previous processes. In FIG. 3B, a sacrificial oxide layer 304 is formed by thermal oxidation. Due to the large quantity of carbon contamination 302 which inhibits the growth of oxide, the sacrificial oxide layer is very thin, for example, as thin as 100 .ANG.. In FIG. 3C, using diluted hydrogen fluoride as an etchant, the sacrificial oxide layer 304 is removed. Since a part of the carbon contamination 302 is trapped by the sacrificial oxide layer 304, this part of carbon contamination is removed with the sacrificial oxide layer 304. However, as shown in the figure, the sacrificial oxide layer 304 does not trap all the carbon contamination 302, so a residue of contamination 302a still remains on the substrate. In FIG. 3D, a gate oxide layer 306 is formed on the substrate 300 containing the carbon contamination residue 302a.
A residue of carbon contamination on a semiconductor substrate appears inevitable when using the above two methods. Thus, to remove the carbon contamination completely, hydrogen fluoride can be used as an etchant to perform an etch process for a long time, but then the devices or other structures are easily damaged due to corrosion from the hydrogen fluoride etchant. There is thus a need for a way to remove the carbon contamination, preferably quickly, while not damaging the other devices and structures on the substrate.